Modern production of semiconductor chips and also of surface acoustic devices, thin-film magnetic heads and similar devices is done by steppers. Modern steppers use four times reduction, excimer lasers as illumination source and use a step-and-scan principle where the wafer and reticle are scanned during exposure. In this context any type of stepper will be referred to as a stepper or as a scanning stepper where the distinction is important.
The important property of a stepper is that it produces identical copies of the same die by exposing the same mask at each exposure site. By doing so in an efficient way it provides high through-put and production economy. There are situation and cases where the chips should not be identical, such as for parameter try-out in research and development (R&D). The manufacturer is then forced to complicated procedures, like writing part of the pattern in an e-beam pattern generator or performing focussed ion-beam modification of the devices.
In other cases there is a need to provide a unique signature or code or programming to each chip. A common way to include such codes is by using electronically programmable circuits, often by changing reversibly or irreversibly the potential of a floating gate electrode, or by programmable fused links or so called anti-fuse links. In either case there are extra process steps, extra manufacturing cost-and/or extra driving involved.